The present invention relates to a digital watermarking technology for embedding information in still images, moving picture (video) images, and other types of content.
The need to ensure copyright protection of still pictures, video content, and other digitally recorded content has driven recent advances in digital watermarking technology. Digital watermarking is a technology for embedding a digital watermark in content data, which is typically accomplished by changing a feature value of a specific part of the content data according to predefined rules for each bit of the digital watermark bitstream. With this method the digital watermark cannot be extracted from the content data without using at least the above rules. For example, if information about the creator (author) of the image data is embedded as the digital watermark information to a location in the image data determined according to the above rules in a format whereby the digital watermark is not visible, the author of the image data can be identified by extracting this information from the place defined according to said rules in the image data.
The principle used by a conventional digital watermarking technology to embed information in image data is described below with reference to FIG. 7.
Referring to FIG. 7, an embeddability analysis process (further described below) is applied to the original image data 500 to generate an embedding strength map 501. This embedding strength map 501 indicates, for each part of the original image data 500, how much a selected feature value (such as luminance) can be changed to embed the digital watermark in a particular part of the original content data (step S5001).
The bits of the watermark data 502 bitstream are then arranged to the parts in the original image data selected according to the predefined rules, and a watermark pattern 503 is generated (step S5002). Note that this map uses +1 to indicate an arranged bit, with a bit value of 1, −1 to indicate an arranged bit with a bit value of 0, and 0 to indicate where no bit is arranged.
The embedding strength map 501 and watermark pattern 502 are then multiplied together to produce watermark pattern 504. Watermark pattern 504 indicates the change in the selected feature value at each location, and is used to embed each bit of the watermark data 502 to a corresponding location in the original image data (step S5003).
The original image data 500 and watermark pattern 504 are then merged. This changes the feature value of each part (image area) of the original image data where the watermark is located with the amount of change determined by the watermark pattern 504 data for that part (image area) (step S5004).
The embeddability analyzing process used in step S5001 above is described next. As noted above, the embeddability analyzing process is a process for generating an embedding strength map, which is a table of values obtained by measuring, for all parts of the content data, the allowed change in the feature value of the parts that can be used for embedding a watermark. For simplicity below let the content data be image data, and luminance be the feature value used for embedding bit values.
The allowed change in the feature value in this example is the amount of change in the luminance (that is, a gradation change) that will not be visible to the eye. Referring to FIG. 7, experience has demonstrated that a large change in luminance is not visibly conspicuous in image areas A with a large high frequency component (such as dark hair), but is visibly obvious in areas B where there are few high frequency components (such as the cheeks) and in contour areas C. It is further known that while the luminance change is great in all directions in image areas where the high frequency component is large, there is little change in the luminance value in all directions where the high frequency component is small, and while the luminance change is great along vectors crossing contours, there is little luminance change along vectors following the contour. These characteristics can therefore be used to detect the luminance change on a plurality of predetermined vectors for each part of the image data, and set the allowed change in the feature value for each said part so that the allowed change becomes smaller as the smallest change in the feature value becomes smaller.
Embeddability analyzing processes are described more fully in the following paper written by the present inventors and others, “Method for maintaining digital watermark image quality based on contour preservation” (in Japanese), Echizen Isao, Yoshiura Hiroshi, Anzai Kohzuke, Taguchi Jun'ichi, Kurozu Yutaka, Sasaki Ryoichi, Tezuka Satoru, Transactions of the Information Processing Society of Japan, Vol. 41, No. 6, pp 1828-1839, 2000.
Conventionally, the sequence of steps S5001 to S5004 shown in FIG. 7 is performed as a single process for embedding digital watermark data in image data. That is, all steps of this embeddability analyzing process are performed each time a digital watermark is embedded in the image data.
When delivering or distributing content data to plural users, the need to detect illegal copies has in particular made it desirable to embed user information (that is, information about the purchaser to whom the content data is delivered or distributed) in the content data. This means that the embeddability analyzing process according to the related art as described above is performed for the same content data every time the content data is distributed or delivered. As further noted above, this embeddability analyzing process includes a procedure for measuring the allowed change in a particular feature value for each part or area of the content data. This means that the processing overhead of the pattern generating process, for example, is high. Repeating this process the same number of times the content data is distributed or delivered simply makes the processing load that much greater.
There is also the desire to embed watermark data in real time to image data captured with a video camera for live broadcasting, for example, so that the live video is broadcast with the watermark embedded in the signal. Copyright protection concerns make it further preferable to embed this watermark data in each frame of image data output sequentially from the video camera. However, the high overhead of the above-described embeddability analyzing process and need for compatibility with extant systems means that an extremely high priced, high performance processing apparatus is needed by the related art.